Electronic components utilizing integrated circuit chips are used in a number of applications. Controlled Collapse Chip Connection (C4 or “flip chip”) is an interconnect technology developed by IBM that provides a large chip to package I/O capability. Typically, one or more integrated circuit chips are mounted above a single or multiple layer substrate and pads on the chip are electrically connected to corresponding pads on the substrate by a plurality of electrical connections such as solder bumps.
The die-attached flip chip package typically includes a gap or space between the integrated circuit chip and the substrate resulting from the overall height of the solder bump connection between the chip and the substrate or adjacent carrier. The substrate material and/or the interconnect solder can emit alpha particles. Alpha particle emissions result from the radioactive decay of impurity elements such as polonium, thorium and uranium in the substrate or the interconnect solder. The alpha particles can cause changes in both logic and memory functions of the device in close proximity to the alpha particle source. Such a disruption of the normal operation of the semiconductor devices is commonly referred to as a soft error upset (SEU) and the overall contribution to the performance of the device is described as the soft error rate (SER).
The C4 joints are susceptible to cyclic fatigue cracking due to the expansion mis-match between the chip and the underlying substrate, or carrier. In order to improve the reliability of the solder joints under cyclic field operating conditions of the package (on/off), the gap is often filled with an underfill material. The underfill material is often a particle-filled epoxy compound that enhances the fatigue life of the C4. The underfill material also helps to absorb the alpha particle radiation. The fillers in the epoxy underfills should be of high purity to assure that the underfill itself is not a source of alpha particles.
There are, however, packages that do not contain the traditional epoxy underfill and hence are susceptible to SER. These packages are typically large multi chip modules (MCMs) that require chip reworkability for upgrades or defective chip replacement. Removing the hardened, or cured underfill (such as the conventional epoxy underfills) for chip rework is complicated, if not impractical. There are also other packages which do not contain the epoxy underfill such as those that do not suffer from the expansion mis-match between the chip and the carrier, or those that are not subjected to large cyclic temperature excursions in field operation.
Earlier MCM packages contained thin film technology, a thin film of metal and/or a polymer such as a polyimide on the substrate, which provided the unintended benefit of absorbing the alpha radiation from the substrate. However, thin film technology tends to be very expensive. As described earlier, the use of hardened, or cured epoxy underfill makes device rework extremely difficult and economically unfeasible. Therefore, there is a need to provide an effective alpha particle barrier that is easily reworkable and cost efficient, and which minimizes or eliminates SER.